To purify a well by means of permanganate of potassium, take from 2 to 4 ounces of the chemical, according to the size of the well, the larger quantity being required only in the case of the enormous wells 10 to 12 feet wide that are occasionally met with; draw a bucketful of water and dissolve the permanganate in its contents by stirring with a stick. Lower the solution into the well, and flounce the bucket about in the water till the permanganate is thoroughly mixed. Permanganate attacks the organic matter present in the water, on which the disease germs and other micro-organisms feed, and so kills them by starvation, besides which the brown precipitate which is formed carries down with it much suspended matter. This process is of especial value in destroying the cholera microbe, and when moving about in camp, it is an excellent precaution to send on a man two days in advance to disinfect in this way the wells that will be used in each camp. If the amount of permanganate used be sufficient, the water should still retain a faint pink tinge after twenty-hours, and should have another day’s rest to settle before being again taken into use. The purer the water, the longer will the pink tinge persist, while on the other hand, the rapid disappearance of the colour is an indication of great foulness, and of the necessity for the application of a further supply of permanganate. The water of large rivers is usually fairly reliable, provided the water be got from the full current and not from a backwater, but is often very turbid from fine sand and other mineral matter. In such cases, the water may be cleared by stirring it round with a crystal of alum; after which the suspended mineral matter will sink to the bottom in the course of an hour or so. The manner in which the alum acts is not clearly understood, as the amount dissolved is so small as to be insufficient to affect the taste of the water or to do any harm, even if it be consumed for a long period. The action of the alum, too, is far more efficient than that of any ordinary filter, completely clearing glacier-fed water turbid with particles of such extreme fineness that they will pass anything except the biscuit-porcelain, or Pasteur, filter. Alum may also be used for disinfecting wells when permanganate is not obtainable, about twice the weight of alum being used, but it is not as reliable; and in case of need, lime may be used, 40 or 50 lbs. of well-slaked lime being thrown into the well, and thoroughly mixed with the water by keeping it disturbed for some time.